Ultrasonic tomography for in-process measurements of temperature in a multi-phase medium

ABSTRACT

A method and apparatus for the in-process measurement of internal particulate temperature utilizing ultrasonic tomography techniques to determine the speed of sound through a specimen material. Ultrasonic pulses are transmitted through a material, which can be a multi-phase material, over known flight paths and the ultrasonic pulse transit times through all sectors of the specimen are measured to determine the speed of sound. The speed of sound being a function of temperature, it is possible to establish the correlation between speed of sound and temperature, throughout a cross-section of the material, which correlation is programmed into a computer to provide for a continuous in-process measurement of temperature throughout the specimen.

The United States Government has rights in this invention pursuant toContract No. DE-AC07-76ID01570 between the U.S. Department of Energy andEG&G Idaho, Inc.

BACKGROUND OF THE INVENTION

The present invention relates to a method and apparatus for thenon-invasive measurement of internal particulate temperature utilizingultrasonic tomography techniques.

The food processing industry currently utilizes two different processesfor the sterilization and preparation of food products. The first methodis commonly referred to as the conventional canning method and has seenlittle change in nearly 100 years. In the conventional canning method, acontainer is filled with food materials and sealed. The next step is theretort which is commonly performed in batches. The retort isaccomplished by heating the food material to temperatures in the rangeof 120° to 150° C. (250° to 300° F.) and maintaining the temperature forspecified periods of time, typically 5 to 15 minutes. Although theretort step cooks the food, its primary purpose is the destruction ofharmful organisms, most notably those responsible for botulism. Thetime/temperature requirements for a particular food material isdetermined experimentally for each plant and process, but is alsosubject to a number of practical constraints which introduce the needfor safety factors.

After retort the food product is cooled and a representative number ofcontainers are punctured to determine the internal temperature by theuse of a thermometer or thermocouple. The temperature measurement doesnot assess the internal particulate temperature, but only serves toverify that the process parameters are within acceptable limits. Inorder to assure that the particulate temperature has been elevated andmaintained for an adequate period of time, extremely conservative safetyfactors are added to the process, such as, processing at a highertemperature or for a longer period of time, or both. As a result,product quality is degraded by overcooking, and a substantial amount ofenergy is wasted in the process. Additionally, containers that have beenpunctured must be disposed of at a significant cost to the foodprocessing industry, costs which are ultimate borne by the consumers.

Another method of processing food is called the aseptic process, inwhich the food materials are pumped continuously through a heatexchanger in order to raise the temperature of the material to apredetermined level and maintain the temperature for a prescribed periodof time. The food product is then cooled and finally placed and sealedin pre-sterilized containers. The aseptic process typically uses 10 to20% of the energy of the equivalent conventional process and produces abetter quality product because it does not overcook the product as theconventional canning method does. However, in the United States theaseptic process is only used for products consisting of a single phase,for example, fruit juices and other liquids.

In single phase materials, temperatures can be measured simply andaccurately at any desired location in the heat exchanger bythermocouples or similar devices. However, the residence time in theheat exchanger is not well defined since the product which travelsthrough the heat exchanger moves at different speeds, depending upon itsproximity to the walls of the heat exchanger. The aseptic process hasnot yet been approved by the Food and Drug Administration for foodmaterials containing particulates, i.e., multi-phase materials, due tothe uncertainties of the temperature of the particulates in the foodmaterial and the residence time.

Ultrasonic temperature devices are known which have the capability forthe non-invasive measurement of temperature in a single medium. Thesedevices are based upon the principle that the speed of sound in anymedium is a function of temperature. In a single phase medium, athrough-transmission measurement is performed using one acoustictransducer as a sending element to generate short pulses of sound, and asecond transducer to receive the sound pulses. The transmission timebetween the two is a measure of the speed of sound.

However, when measurements are made on two (or multi-) phase media, theproblems involved in relating the measurements to the physical systemare far greater. (see C. Javanaud, "Applications of Ultrasound to FoodSystems", Ultrasonics, vol. 26, pp. 117-123, May 1988). The problemarises because the speed of sound at a given temperature is a uniquefunction of each material. In general, the speed of sound is differentin one material as opposed to another, even when the two materials areat the same temperature. When the acoustic flight path of an ultrasonicpulse is partly in one medium and partly in another, for example liquidand solid, the time of flight is proportional to the average speed ofsound in each medium along the flight path. If the location and/or thedimensions of the solid or semi-solid particles along the transmissionpath are not known, a unique value for the speed of sound through theparticle cannot be determined.

Therefore, it is necessary to know the length of flight in each medium,so that the speed of sound and thus the temperature in each medium isknown from the overall measurement of the transit time of the ultrasonicpulse. Tomography techniques can be used to determine the length offlight in each medium, and therefore the temperature at a specific pointin a non-homogeneous material. The use of ultrasonic tomography forin-process measurements of particulate temperatures in the foodprocessing industry will result in lower energy requirements, reducedcosts associated with disposal of unusable product, and will result in abetter quality product since overcooking is avoided. Additionally,ultrasonic tomography can be used by the food processing industry fordetermining the in-process particulate temperatures in freezingoperations, thereby reducing energy requirements and costs.

It is an object of this invention to provide a device that measures theinternal temperature of particulates during processing.

It is another object of this invention to provide a device that measuresthe temperature of a multi-phase material at any location within thematerial.

It is another object of this invention to provide a method fornon-invasively determining the temperature in a multi-phase material.

Additional objects, advantages and novel features of the inventions willbecome apparent to those skilled in the art upon examination of thefollowing and by practice of the invention.

SUMMARY OF THE INVENTION

To achieve the foregoing and other objects, this invention comprises amethod and apparatus for determining, by non-invasive means, thetemperature of a particle in a multi-phase medium, utilizing speed ofsound tomography techniques. A multiple number of ultrasonictransducers, which function as ultrasonic transmitters and/or receivers,are positioned around the material for which the temperature is desiredto be known. The transducers are positioned so that all cross sectionalgrid system cells are traversed by at least two ultrasonic pulses. Themultiple number of transducers emit ultrasonic pulses through themedium, and the transit time that it takes the pulse to travel throughthe medium is measured. From the overall transit times of the pulsesthrough the medium, it is possible to calculate the speed of soundthrough any given cell within the grid. The temperature can then bederived for a specific cell location based upon previously determinedcharacteristics between the speed of sound and temperature for thatparticular material.

BRIEF DESCRIPTION OF THE DRAWINGS

For the purpose of facilitating an understanding of the invention, thereis illustrated in the accompanying drawings a preferred embodimentthereof, from an inspection of which, when considered in connection withthe following description, the invention, its construction andoperation, and many of its advantages should be readily understood andappreciated.

FIG. 1 depicts the potential locations of ultrasonic transducers arounda container (e.g. a pipe, can, etc.) of the medium to be measured. FIG.1 also shows an example of a grid system within the medium.

FIG. 2 shows possible flight paths for the ultrasonic pulses between thetransducers.

FIG. 3 shows the flight path of an ultrasonic pulse through a particularcell of the grid system.

FIG. 4 is a graph showing the transit time characteristics for soundthrough water at a given temperature.

FIG. 5 is a graph showing the transit time characteristics of soundthrough a solid (i.e., a potato) at a given temperature.

FIG. 6 is a graph showing the speed of sound through a potato immersedin water having a temperature of 28° C.

FIG. 7 is a schematic diagram of the preferred embodiment of the presentinvention.

FIG. 8 is a graph representing the speed of sound in a multi-phasemedium with no modelling noise.

FIG. 9 is a graph representing the speed of sound in a multi-phasemedium with 0.05% random modelling noise.

FIG. 10 is a graph representing the speed of sound in a multi-phasemedium with 0.10% random modelling noise.

DETAILED DESCRIPTION OF THE INVENTION

The use of tomographic techniques provides a method for determining theeffective flight path in each medium of a multi-phase media. Thetomographic process is capable of providing a map or image of the crosssection of the media being imaged, from which the speed of sound in anysmall portion can be obtained. However, a map or image is not necessaryif the length of the flight path is known and the time of flight ismeasured. From these two known variables it is possible to calculate thespeed of sound through any particular segment of the medium. It isunderstood that the speed of sound is a function of, inter alia,temperature. Once the speed of sound is known for each segment of themedia, the temperature for that segment can be calculated usingpreviously derived characteristics of sound and temperature for thatparticular medium.

Referring now to the drawings in which like numerals represent likeelements throughout the several views, the preferred embodiment of thepresent invention will be described. FIG. 1 shows a possible arrangementof ultrasonic transducers 10a-10j around the perimeter of a container 14of material for which the temperature is desired to be measured.Transducers 10a-10j can function as both transmitters and receivers ofultrasonic pulses, or the transducers can function strictly as atransmitter or receiver of the pulses. The perimeter of the containercan be a pipe through which the material flows, or it could be someother form of container, such as a metal can or plastic bottle. FIG. 1also depicts an example of a mathematical grid system within thecontainer 14 wherein the grid is composed of small sectors (cells)covering the entire cross section of the specimen material. Each cellrepresents an area or volume over which the speed of sound is averaged.The quality of the tomographic image, and of the measurement of thespeed of sound, depend upon the size of each cell. The image andmeasurement improve as the cells become smaller, however, smaller cellsresults in more ultrasonic transducers, additional electronics andgreater sensitivity to background "noise". The ultrasonic transducers10a-10j are arranged so that at least two ultrasound flight paths willtraverse every cell in the grid system.

FIG. 2 shows the flight paths of the ultrasonic pulses that are to beemitted and received by transducers 10a-10j. In the preferred embodimentof the present invention, the frequency of the ultrasonic pulses is therange of 20 kHz to 100 MHz. Depending upon the properties of thespecimen to be measured, the frequency of the ultrasonic pulses could bevaried. By superimposing the ultrasonic pulse flight paths onto the gridsystem shown in FIG. 1, it is possible to determine the distance of eachflight path through every cell of the grid system, as illustrated inFIG. 3. With the flight path distance through each cell known, the nextstep is to measure the transit time of the ultrasonic pulses through themedia. The transit time of various pulses traversing each cell isassumed constant in that cell.

The next step is to combine the measurements in the mathematical processcalled tomography to produce the equivalent of an image of the region,if desired, through which the sound beams passed. The mathematicalprocess is a series of linear simultaneous equations of the matrix form:

    [D.sub.nm ]*[1/T]=[S]

where:

D_(nm) =the ultrasonic flight path distance of the nth pulse across eachmth cell,

1/T=the inverse transit time of each ultrasonic pulse, and

S=the speed of sound through each cell.

These equations are solved for the speed of sound in each cell, fromwhich a corresponding temperature can be determined based uponpreviously derived characteristics between the speed of sound andtemperature for the particular medium. Other algorithms could also beused to solve for the speed of sound in each cell.

Individual transit time measurements can easily be made to 0.01% or lessunder industrial conditions. Uncertainties of this order correspond toerrors in temperature measurement of tenths of a degree or less. FIG. 4illustrates the transit time characteristics for sound in water at anygiven temperature. These characteristics are derived throughexperimental procedures for each medium. FIG. 5 shows the transit timecharacteristics for sound in a solid, in this case a potato, at anygiven temperature. FIG. 6 shows in greater detail the transit time ofsound through a solid (i.e., a potato) which is immersed in water havinga temperature of 28° C. Again, these characteristics are derivedexperimentally.

FIG. 7 is a schematic diagram showing the preferred embodiment of thepresent invention. As shown in FIG. 7, the operation of the presentinvention is straightforward, requiring little operator training orintervention in the process. The computer sends a signal to a signalconditioning unit which includes electronic circuitry for pulsegeneration, receivers, and filters. Through the signal conditioning unitthe computer signal triggers the sequential transmission of ultrasonicpulses from the ultrasonic transmitting transducers 10a-10j. Theultrasonic pulse travels through the specimen and is received by areceiving transducers 10a-10j which transmits an analog signal to thesignal conditioning unit circuitry to prepare the signal for analog todigital conversion. The digital signal is then transmitted to thecomputer. The computer measures and records the flight time of eachultrasonic pulse through the specimen. The predetermined characteristicsbetween the speed of sound and the temperature for the medium beingmeasured have been programmed into the computer. Based upon the knownflight path and the measured time of flight, the computer will thencalculate the speed of the ultrasonic pulses through each cell. Thecalculated speed of sound through each cell can then be used to providea tomographic image of the specimen by, for example, assigning a greyscale to for each measured speed of sound. In the preferred embodimentof the present invention, the tomographic image is not necessary todetermine the internal temperature of the particulates. However, it maybe desirable to have the ability to provide an image of the specimen foradditional data that may be desired, such as the density distribution ofthe product. The computer will then compare the calculated speed ofsound with the programmed temperature operating limits. The computer canalso be programmed to provide process controls in the event thetemperature is determined to be outside of the established range ofacceptable temperatures for that medium. The process controls couldinclude, for example, increasing the residence time in a heat exchangeror increasing the heat transfer rate.

EXAMPLE

Applicant conducted a number of experiments to verify the operation ofthe subject invention in multi-phase materials. In the experiments themulti-phase material was comprised of water and a solid, the solidtypically being different varieties of potatoes. Food materials vary inthe amount of water content and potatoes are believed to berepresentative of most food materials in this respect. The sound ispropagated primarily through the water component of the food material'sstructure, but the propagation is modified by scattering and otherinteractions with the solid part of the structure. Potatoes areconsidered a worst case among food materials from the viewpoint ofchanges in structural properties induced by temperature, since thestarch content of potatoes is a major constituent that is known tochange when heated, thus changing the speed of sound through the potato.

Typical food processing techniques involve temperatures above 120° C.and therefore a pressurized apparatus was utilized in the experiments toachieve these temperatures. Coaxial cables for ultrasonic transducersand a thermocouple were passed through the boundary of the pressurizedapparatus. The transducers were positioned over an adjustableexperimental path, the length ranging from 25 mm to nearly 125 mm. Thethermocouple was placed in the center of the food material to measuretemperature. An external pressure gauge permitted a thermodynamicdetermination of the temperature of the water within the pressurizedapparatus.

The test sensors were special high-temperature ultrasonic transducersoperating at a nominal 750 kHz center frequency. This frequency waschosen from experience with difficult to penetrate materials often foundin nondestructive testing applications. The effective diameter of thesound beam was approximately 15 mm.

For calibration, the pressurized apparatus was completely filled withwater only. Measurements were made relative to the sound pulse transittime through the reference material (water) at a reference temperature(nominal 20° C.) to provide calibration for inter-transducer spacing. A65 mm transducer spacing is a typical calibration. As shown in FIG. 4the ultrasonic pulse transit time shows a maximum speed of sound at 73°C. with a measurement precision of ±0.22° C.

Ultrasonic sensitivity of the system over the temperature range showedonly a slight change, dropping less than 5% at 115° C. In water, amedium with low ultrasonic attenuation, it was observed that both thethrough-transmission signal and pulses make one or more round tripsbetween transducers. Up to 12 such multiples were observed in theseexperiments, indicating good signal/noise ratio and transduceralignment.

Subsequent measurements were made with the solid (i.e. potato) in place,over a temperature range of 20° to 120° C. Transducer placement was suchthat the external water acted solely as a heat transfer medium, theeffective sound path was entirely in the solid. FIG. 5 representstypical transit time results relative to the temperature for potatoes.

From these experiments it was observed that the speed of sound in thepotato was a few percent higher than that of water at any temperatureand that the attenuation of the sound beam in the potato does not appearto be a significant problem at any temperature. It was also observedthat starch gelling hardened the food material structure, therebyraising the speed of sound, but this change was completed attemperatures well below those of interest in food processing. The curveof speed of sound versus temperature for potatoes approximatelyparallels that of water above about 110° C., and has adequate slope andreproducibility to permit temperature measurements of good resolution.

It was further observed that thermal gradient effects indicated aslightly lower temperature for the potato than would otherwise be thecase. However, specimen size would be smaller in commercial applicationand the sound beam diameters would also be smaller, both of which wouldreduce the effect of thermal gradients. A permanent increase in transittime, amounting to one to two percent, was found in all samples uponcooling to room temperature after heating. This is due to a hardening orstrengthening of the solid structure by the gelling of starch. Nosignificant differences between potato varieties were noted.

Tomographic reconstruction can be performed using a large number ofknown algorithms. Each has its own advantages for one or moreapplication, but there is not known general analytical method fordetermining the best algorithm for all applications. Applicant'sapproach, therefore, was to perform a number of numerical "experiments"by building and varying a mathematical model of a typical sensor systemwhich can be used to derive guidelines for the sensor design.

Any tomographic reconstruction algorithm has a certain amount of what iscalled "modelling noise" associated with it. The modelling noise isrelated to the fact that the reconstruction squares, or otherdescriptions on which the image is based, are finite and the shape of anarbitrary object cannot be exactly represented by any combination ofsquares. The smaller the square, the less modelling noise results,however, the image also becomes more susceptible to the effects of realelectronic and measurement noise. Spatial resolution, the smallest areaover which the speed of sound is averaged to obtain its temperature,follows the same general rules as modelling noise. Spatial resolutionaffects the sensitivity of the measurement of the speed of sound andtherefore the accuracy of the temperature measurement.

The numerical experiments first addressed the way in which modellingnoise, which is an inherent property of every tomographic algorithm,changes with the parameters of the mathematical model for the numbers ofultrasonic transducers that have the physical size and geometry of apipe sensor. Then spatial resolution was calculated for models,algorithms and ultrasonic pulse combinations, which tended to minimizemodelling noise. Finally, Monte Carlo calculations were used to estimatethe effect of measurement noise. The result is a series of curves asdepicted in FIG. 8, FIG. 9 and FIG. 10. In this sequence, the MonteCarlo calculation randomly placed five circular solid objects in abackground of water, wherein the speed of sound in the solid was 1% overthat of the background. This approximated, for calculational purposes,the characteristics of potatoes in water that were previouslyestablished from experiments. The figures are histograms representingthe number of grid squares in which the model calculated a speed ofsound. Each figure represents the results of five hundred differentrandom placements of the five circular solid objects in the measurementgrid.

As shown in FIG. 8, the width of the "spikes" representing water, at avalue of 1.0 for the speed of sound, and "potato" at 1.01 are measuresof the modelling noise. FIG. 9 shows the effects of an addition of 0.05%random electrical noise and FIG. 10 represents the random addition of0.10% electrical noise. From these figures it is apparent that modellingnoise can be minimized to within the limitations of known algorithms andultrasonic transducer characteristics and numbers.

The foregoing description of a preferred embodiment of the invention hasbeen presented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed, and obviously many modifications and variations are possiblein light of the above teaching. The embodiments described explain theprinciples of the invention and practical application and enable othersskilled in the art to utilize the invention in various embodiments andwith various modifications as are suited to the particular usecontemplated. It is intended that the scope of the invention be definedby the claims appended hereto.

The embodiments of this invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A method of determiningthe temperature throughout a multi-phase medium on the basis of speed ofsound tomography, the steps comprising:establishing sectors throughout across-sectional portion of a multi-phase specimen; positioningultrasonic transducers around the specimen such that multiple ultrasonicpulse flight paths traverse each established sector; transmittingmultiple ultrasonic pulses by said ultrasonic transducers through eachsector of the specimen; receiving said ultrasonic pulses which havetravelled the established flight paths through said sectors of thespecimen; identifying, measuring and recording the transit time of eachultrasonic pulse through said specimen; calculating the speed of soundthrough said sectors of the specimen; correlating a temperature in saidsectors of the multi-phase specimen to said calculated speed of soundbased upon established characteristics between speed of sound andtemperature for said specimen.
 2. The method of claim 1 wherein saidultrasonic pulse is in the frequency range of 20 kHz to 100 MHz.
 3. Themethod of claim 1 wherein a computer is used to control the transmissionand receipt of said ultrasonic pulses.
 4. The method of claim 1 whereinsaid identifying, measuring and recording of the transit time of eachultrasonic pulse is performed by a computer.
 5. The method of claim 1wherein a computer is used to calculate the speed of sound through saidsectors, said computer being programmed to correlate a temperature tosaid speed of sound for said specimen.
 6. An apparatus for performingthe method of ultrasonic tomography for in-process measurements oftemperature in a multi-phase medium, comprising:a. transducer means foremitting multiple ultrasonic pulses through sectors of a multi-phasemedium specimen; b. transducer means for receiving said multipleultrasonic pulses; c. means for measuring and recording the transit timeof said ultrasonic pulses through said sectors of the multi-phase mediumspecimen; d. means for determining the speed of sound through eachsector and correlating said speed of sound to a temperature for anysector in the multi-phase medium specimen.
 7. The apparatus of claim 6wherein said transducer means for transmitting and receiving saidultrasonic pulses is controlled by a computer.
 8. The apparatus of claim6 wherein said means for measuring and recording transit times of saidultrasonic pulses is a computer.
 9. The apparatus of claim 6 whereinsaid means for determining the speed of sound and correlating said speedof sound to a temperature is a computer which has been programmed withpredetermined characteristics of the speed of sound and temperaturethrough said specimen.
 10. The apparatus of claim 6 further comprisingmeans for the tomographical imaging of the speed of sound through saidmulti-phase medium specimen.